1. Technical Field
This invention relates to the art of cold extrusions of externally toothed helical members, such as helical gears, and more particularly to techniques for extending the fatigue life of dies used to carry out the extrusion.
2. Discussion of the Prior Art
The original process to cold form helical external gear teeth to near net shape was developed during the early 1970's (see U.S. Pat. No. 3,910,091) and was initially used to make planet pinions useful in automatic transmissions for automotive and truck applications. This process replaced slower and more costly conventional machining of gear teeth such as by metal cutting hobbing methods.
To produce more acceptable workpiece quality, including less shape variability and greater die life, the die must have internal teeth which have a contoured entrance face and relatively thin die tooth tip thickness, which tip is linearly relieved at a trailing position (such as disclosed in U.S. Pat. No. 4,622,842). Unfortunately, such internal teeth configurations provide for an acceptable die life (meaning greater than 50,000 or more extruded blanks through such die) only if the aspect ratio (height/width ratio) of the tooth profile remains less than about 1.5. At greater aspect ratios, the die life deteriorates rapidly due to excessive torsional loading and bending of the tooth tip at the beginning or transitional zone of the relieved portion, thus causing breakage. High aspect ratios are desirable in many new gear designs because they promote a quieter, smoother action and thus this problem must be overcome.
It is an object of this invention to reduce the bending load on internal die teeth having large aspect ratios and an accompanying thin die tooth tip; the reduction should take place by directing the loading to the stronger tooth base away from the thin tooth tip or vulnerable land-to-relief transition zone.